Tickets to the Asifa at Citi Field and Arthur Ashe Stadium were issued through the Ticketmaster system. So perhaps the Siyum tickets at Metlife were as well.
Also, the fact that they added over 10,000 seats in addition to the normal seating capacity at Metlife, indicates they sold out. Plus all the Police agencies in the Tri-State Area are aware of the massiveness of the event.